Elections to Castlereagh Borough Council were held on 17 May 1989 on the same day as the other Northern Irish local government elections.

The election used three district electoral areas to elect a total of 21 councillors.

Note: "Votes" are the first preference votes.

1985: 4 x DUP, 2 x UUP, 1 x Alliance 1989: 4 x DUP, 2 x UUP, 1 x Alliance 1985-1989 Change: No change 1985: 3 x DUP, 3 x UUP, 1 x Alliance 1989: 3 x DUP, 1 x Alliance, 1 x UUP, 1 x UPUP, 1 x Independent Unionist 1985-1989 Change: UPUP and Independent Unionist gain from UUP (two seats) 1985: 3 x DUP, 3 x UUP, 1 x Alliance 1989: 3 x UUP, 2 x DUP, 2 x Alliance 1985-1989 Change: Alliance gain from DUP